Admittedly this is not a very intuitive error message (and it does deserve some
attention) but the reason for this is that the °».°… operator is no longer
required for °»recursive template instantiations°…. So instead of:
const int birdofprey = hawk!(.eagle!());
try
const int birdofprey = hawk!(eagle!());
instead!

Looks like a debug message got left in :-)
Doesn't happen if you remove the dot.